A friend of mine went to the last Orshabaal that appeared in Collabra, he said that the boss lasted about 15 turns and his total damage was approximately 1000. This means that the boss died while he was still resistant. In theory, this should mean that your damage during that period does count. However, I have seen complaints from players who claim that they participated in the fight and did not even get the kill in the bosstiary.
CipSfot never gave many details about how the participation system works, and with this change some adjustments may still be necessary. My tip is: in addition to focusing on the attack, try to take damage and, if possible, heal other players.